This application relates to methods of preparing amino acid chelates which are essentially free of contaminating anions. More particularly, this invention relates to methods of preparing pharmaceutical grade amino acid chelates. This application is related to copending application Ser. No. 738,065 filed May 24, 1985 entitled "Pure Amino Acid Chelates", which will issue July 8, 1986 as U.S. Pat. No. 4,599,152.
Amino acid chelates are products resulting from the reaction of a polypeptide, dipeptide or naturally occurring alpha amino acids with a metal ion having a valence of two or more to form a ring structure wherein the positive electrical charges of the metal ion are neutralized by the electrons available through the carboxylate or free amino groups of the alpha amino acid. For convenience sake, metal ions having a valence of two or more will simply be referred to as divalent metal ions or divalent cations.
Chelate formation through neutralization of the positive charges of the divalent metal ions can be through the formation of ionic, covalent or coordinate covalent bonding. In the past, amino acid chelates have generally been made by first dissolving a water soluble divalent metal salt in water. An amino acid ligand is then reacted with the metal ion at a ratio of ligand to metal of at least 1:1 and preferably at least 2:1. Often, the ligand is a hydrolysis product obtained by acid, base, base-acid, or base-acid-base hydrolysis. In such cases, the by products from hydrolysis, such as anions including chlorides, sulfates, phosphates and nitrates, and cations, including potassium and sodium remain in the hydrolysate. Reaction products of metal salts with proteins or with acid and/or base hydrolyzed proteins are taught in U.S. Pat. Nos. 2,960,406 (Cardon); 3,396,104 (Miller); 3,463,858 (Anderson); 3,775,132 (Richards); 4,020,158 (Ashmead et al); 4,103,003 (Ashmead) and 4,172,072 (Ashmead). In order for the reaction to proceed to completion with the formation of a cyclic chelate ring, the amino acid has had to be at a pH which is preferably above, or more basic than, the isoelectric point of the amino acid. For that reason, a certain amount of an alkali metal hydroxide, carbonate or bicarbonate has usually been added to the reaction mixture.
Most water soluble salts used in making amino acid chelates have been either sulfates or chlorides. Using the sulfate ion as exemplary, the reaction has generally proceeded as follows: ##STR1## where M is a bivalent metal cation and R is a radical of a naturally occurring amino acid, dipeptide or polypeptide. It is apparent from the above formula that the sulfate anion is present in the reaction mixture in the form of sodium sulfate. U.S. Pat. No. 2,877,253 teaches a product formed by the reaction of one mole of glycine with one mole of ferrous sulfate. That patent indicates that the sulfate anion becomes tied up in the reaction which allegedly forms a ferrous sulfate-glycine complex. Whether or not the sulfate actually participates in the reaction, or is present as the salt of an alkali metal, it nevertheless is present in the reaction mixture. Such products are difficult, if not impossible, to purify. While sodium sulfate, per se, is water soluble, the reaction between a metal sulfate and an amino acid is never carried to 100% completion and the sulfate ion is always present. The same holds true for the presence of chloride ions when utilizing a metal chloride salt for amino acid chelate preparation.
Even if one were to attempt to wash out the excess sulfate or chloride ions with repeated washes, such an attempt could well be counter productive inasmuch as glycine and other amino acid ligands are also soluble to a degree. Hence, depending upon pH, the unreacted ligands or weakly held ligands could also be removed along with the unwanted anions.
Other methods teaching the reaction of metal salts with ligands made up of protein, protein hydrolysates or amino acids are found in U.S. Pat. Nos. 3,168,541 (Hobbs); 3,969,540 (Jensen); 4,167,564 (Jensen); 4,216,143 (Ashmead) and 4,216,144 (Ashmead).
These salt by-products in amino acid chelates cause a variety of problems, not the least of which is cost. When salts are present, it is impossible to obtain an amino acid chelate with as high a metal content as with the salts absent. It also costs both to prepare and ship the unneeded salt. Moreover, these salts often pick up moisture and make handling of the product difficult. Most importantly, the health of humans, plants and animals receiving amino acid chelates is, in general, affected adversely by the presence of these salts.
In U.S. Pat. No. 4,599,152 (Ashmead), electrolytic methods for preparing anion free amino acid chelates are taught. While these methods are successful in preparing such pure forms of chelates, they are relatively expensive and time consuming.